An ELT Glossary : Stress

Stress refers to the pronunciation of a syllable with greater muscular force and using more lung air than that of other syllables in the word or utterance. A listener will perceive the syllable as sounding louder or longer than any unstressed surrounding syllables - ie as having greater prominence.

Stress can be indicated in a variety of ways. Here we will use the convention of using diacritics - a high slash before a syllable with primary stress and a low vertical slash for syllables with secondary stress (see below  for definitions of these terms)

A. Word stress

A word may be composed of just one, or of more than one syllable. Obviously a one syllable word must be stressed on that syllable - eg  Stop! /ˈstɒp/

Rules for stress placement in two- or multi-syllable words do exist, but are extremely complex. For the average learner of English, it is usually easier to learn the stress placement as the word is learnt.

In English, stress placement can sometimes affect word class - for example record will be pronounced /ˈrekɔːd/ as a noun but /reˈkɔːd/  or /rɪˈkɔːd/ as a verb. Other words similarly affected include  desert, export, import, contract, present, extract, increase, decrease,  object, transport. In some of these cases, the meaning of the word  also changes completely - eg desert, object, present (as adjective meaning now).

Multi-syllable words will be composed of a syllable with primary stress, one or more unstressed syllables, and also possibly syllables with secondary stress - ie syllables pronounced with greater force than the unstressed syllables but not so noticeable as the syllable with primary stress. Some examples follow (remember: high slash = primary stress on the following syllable; low slash = secondary stress; no slash = unstressed syllable - so for the first word, organised: /ˈɔː/ = primary stress; /gə/ = unstressed; /naɪzd/ = secondary stress) :

organised           /ˈɔːgəˌnaɪzd/
decorate           /ˈdekəˌreɪt/
academic          /ˌækəˈdemɪk/
understand        /ˌʌndəˈstænd/
information       /ˌɪnfə ˈmeɪʃən/    
consideration    /kənˌsɪdəˈreɪʃən/

Occasionally,  words may have alternative stress placements. The differences may be due to regional varieties (eg US vs UK English) or simply be idiosyncratic and dependent on the individual speaker - who may well use one stress pattern in one utterance and the other in another. For example : 
controversy   may be pronounced as /kən'trɒvəsiː/  or  as /'kɒntrəˌvɜːsiː/

B. Sentence stress

English is a stress timed language, meaning that the rhythm and timing of an utterance are determined by the number of stressed syllables, with the others being “squashed in” between them. This “squashing in” leads to the rapid pronunciation that produces such features of connected speech as vowel weakening, elision etc. Vowel weakening has already been exemplified in the example of controversy above - notice how the schwa (/ə/) is used in the unstressed syllables but not in the stressed syllable - whichever that may be.  Another example was seen in the example of  record . In a sentence like Can you record the news at seven o’clock tonight please? , the /e/ in re... may become the shorter “weaker” vowel /ɪ/, so that the unstressed syllable can be produced more rapidly. Another example of a weak vowel in an unstressed syllable can be seen in the use of the schwa in tonight : /təˈnaɪt/ as well as in seven and o’clock (see the transcription below). 

Many unstressed words in the utterance will, however, be “grammatical” words – articles, prepositions, auxiliary verbs and so on. The grammatical words in the sentence are frequently subject to the greatest amount of reduction – the sentence above might well be pronounced :
/kən jə rɪˈkɔːd ðə ˈnjuːz ət ˈsevən əˈklɒk təˈnaɪt ˈpliːz/
Here, grammatical words like can, you, and at are reduced to their weak form pronunciations as they are in unstressed positions in the sentence.

There may be times however when normally unstressed syllables are given prominence and stressed. This happens eg with grammatical words when they take final position in the clause :
can /ˈaɪ kɑːnt ˈgəʊ bət ˈʤɒn ˈkæn/
you /ɪz ˈðæt ˈrɪəlɪ ˈjuː/
at  /wɒt ə ðeɪ ˈlʊkɪŋ ˈæt/

A sentence like I can’t but John can is also affected by contrastive stress. The word  "Icontrasts with "John"   and is therefore stressed -  ˈI can’t but 'ˈJohn can
although in a non-contrastive sentence (eg I  can’t  ˈgo to ˈnight) it would be unstressed.

When a word is affected by contrastive stress, it also automatically becomes the tonic syllable in the tone group - and is therefore subject to pitch movement as well as simply stress: // \I  ˈcan’t // but '\John ˈcan//

Contrastive stress may not only cause changes in sentence stress, but also in  in word stress. For instance, the words inductive and deductive would normally be stressed on the second syllable :  /ɪnˈdʌktɪv/   /dɪˈdʌktɪv/, but in the case of a misunderstanding, contrastive stress might be used, causing the stress pattern to change : /aɪ ˈsed ən ˈɪndʌktɪv əˈprəʊʧ nɒt ˈdiːdʌktɪv/ Notice again how whether or not the syllable is stressed causes a change in vowel quality – the short vowel /ɪ/being used in the unstressed syllable, but the long vowel /iː/ used when the syllable is stressed.

Emphatic stress also includes prominence on an element that would normally not carry the tonic, but in this case there is no contrast. For example : Oh look! I ˈdo like those shoes  /  I  haven't completely decided but I think I 'will probably go. / You can't ˈreally believe that./  It was nice, but ˈvery cold. / He's ˈso rude.

Further reading